Push-to-talk communication, in which an individual pushes a button to talk to other individuals belonging to the same talk group, has been used and has been popular in Land Mobile Radio (LMR) and Professional Mobile Radio (PMR) networks. However, the provision of a similar service in public cellular networks has been unprofitable in the past, since the implementation of the service in circuit-switched cellular networks would have wasted radio and network resources excessively.
The above drawback can now be eliminated with the help of the GPRS (General Packet Radio Service) technology introduced into cellular networks. This technology, which enables IP (Internet Protocol) packet technology to be introduced into cellular networks, will make efficient implementation of the push-to-talk service possible, thereby making the service available in cellular networks. By means of the GPRS/IP technology, the efficiency of network resource usage can be dramatically improved in situations where a group of people needs to communicate with each other repeatedly, but occasionally. Namely, this technology allows the radio and network resources to be reserved only for the duration of the talk spurts.
The implementation of the new push-to-talk cellular services is based on the IP Multimedia Subsystem (IMS), as specified by the 3rd Generation Partnership Project (3GPP). The IMS integrates mobile voice communications with Internet technologies, allowing IP-based multimedia services to be utilized in mobile networks. The push-to-talk service is implemented using PoC application servers in the IP Multimedia CN (Core Network) subsystem. These servers handle call set-up signaling for the push-to-talk calls, reservation of speaking turns for one user at a time, and routing of the packets to the recipients, for example.
Currently, PoC users must be connected to the GPRS network before a PoC session can be established. In other words, the mobile terminals must move from a detached state (IDLE) to the attached state (READY) in order to be ready for a possible PoC session. However, many users wish to stay in the detached state, i.e. in the mode in which the mobile terminal is not connected to the GPRS network. This is because they fear that if the mobile terminal is constantly connected to the GPRS network, they may receive unwanted GPRS data, which may, in turn, cause unwanted costs or involve security risks. This makes the practical initiation of a PoC session complicated. If the terminals use dynamic IP addresses, as is the case in IPv4 networks, the GPRS network has no technical means for forcing these terminals to the connected mode. Therefore, the person who wants to start a PoC session within a PoC group has to ensure, for example by making phone calls, that the terminals of the users of the group are connected to the GPRS network.
The present invention seeks to eliminate the above-described drawback related to the initiation of push-to-talk services or similar packet-based services in a public mobile communication system.